Mesozoic alkaline dykes in the Sunnhordland region, western Norway: ages, geochemistry and regional significance

The coast-parallel, alkaline, dyke suite in the Sunnhordland region of Norway comprises the youngest igneous event documented in this country. K---Ar measurements on amphibole and whole-rock samples demonstrate at least three episodes of dyke intrusion along recurrently opened fractures at around 275 m.y. 220 m.y. and 160 m.y. There is a progressive trend towards increasing alkalinity and rare earth content and the younger dykes may represent a more fractionated stage or a smaller degree of partial melting at deeper level. The occurrence of ultramafic inclusions and xenocrysts indicates the presence of upper mantle derived material. Intrusion of the dykes is related to rifting, fracturing and differential movement along the margin of the North Sea. The earliest dykes may have been associated with the formation of the West Norway Trough in Permo-Carboniferous time, the main igneous activity with the Hardegsen Pulse in the Early Triassic and the mid-Jurassic activity, which is identical in age to the North Sea volcanic episode, with mid-Cimmerian movements.